1. Field of the Invention
This invention relates to rotary mower vegetation cutting blades of the type that are affixed to a powered rotating support assembly, usually mounted under a protective deck structure, where the blade rotates parallel to the surface to be mowed.
2. Discussion of Prior Art
Powered rotary mowers are used for vegetation control and lawn grooming. To enhance grass health and beauty, a higher cutting height is often desired. Higher vegetation cutting height is especially needed in the event of dry weather should local water conservation ordinances lead to insufficient ground moisture. In order to discharge clippings, present mower blades are designed such that strong airflow is created within the cutting deck. When attempting to cut vegetation at higher heights, contrary to previous blade design intentions, the strong airflow inside the mower deck causes vegetation to lie down. Vegetation that is not standing erect is more difficult to cut. Improving aerodynamic manipulation of the blade induced airflow inside the mowing deck, will enhance cutting and mulching of vegetation at all cutting heights.
Mulching action is somewhat incomplete with present designs, since the clippings to be shredded are accelerated inside the deck and thereby carried along with the airflow. To disrupt the airflow inside the mowing deck, deflectors mounted inside the mower deck have been designed, which are intended to interfere with mowing deck airflow. One such blade is described in U.S. Pat. No. 5,212,938 to Zenner et al. Deck mounted air deflectors only work well in dry conditions. Attempting to mow vegetation that is not completely dry will quickly clog deck mounted air deflectors. Higher costs to produce intricate mowing deck features contribute to the reasons complex mowing deck mulching designs have not been accepted by the marketplace.
Several mower blades designs include raised shredding members such as is described in U.S. Pat. No. 6,487,840 to Turner et al and U.S. Pat. No. 5,291,725 to Meinerding. These blades employ top mounted vertical shredding members designed to rip apart clippings after the primary cutting edge has severed the vegetation from the rooted plant. Mowing deck circular airflow immediately accelerates cut clippings causing the relative speed between the mowing blade mulching members and the vegetation trimmings to be relatively low. Once clippings become airborne, impacts by blade cutting members are less effective since the clippings have little mass and the clippings are traveling in the same direction and speed as the secondary cutting edges.
Blades such as described in U.S. Pat. No. 6,7470,662 to Burke et al and U.S. Pat. No. 5,321,940 to Peterson illustrate rotary mulching blades that cut and shred vegetation featuring the blade leading edge as the primary cutting surface. Cutting vegetation multiple times at multiple heights by multiple cutting edges, while the vegetation is still connected to the rooted plant improves shredding and mulching action, since the vegetation to be shredded held stationary, and thereby impacted more forcefully by the cutting edges.
Blades such as U.S. Pat. No. 6,145,290 to Sullivan employ secondary cutting edges integrated interior to the primary cutting edge which are intended to mulch clippings. A rotating blade's maximum velocity occurs at the area most removed from the rotating vertical supporting assembly axis. Our tests have found that cutting action drops significantly when cutting edges are placed more than approximately four inches (100 mm) from a mower blade's extreme end. Safety regulations limiting the rotational speed of mower blades contribute to the reduction in shredding effectiveness of interior cutting edges. Mowing and mulching efficiency of a rotary mower blade is at a maximum when the cutting edges are designed into the area most removed from the rotational axis, the blade's extreme ends.
Since a rotating blade can impact and forcefully throw objects, safety blades have been designed. U.S. Pat. No. 2,859,581 to Kroll et al and U.S. Pat. No. 3,336,737 to Belfiore describes a circular disk that is intended to push foreign objects aside. While these disk blades may improve safety, these circular blades must use aerodynamic forces to direct vegetation into the path of the primary vegetation cutting edges. Air is a fluid having the properties of viscosity and adhesion which results in atmospheric friction, also called “skin resistance” or “drag.” A circular disk-shaped blade effectively accelerates air by atmospheric friction inside the mowing deck. Strong circular air currents induced by a disk-shaped blade within a mowing deck causes vegetation to lie down which is a feature not desirable in a mower blade design. Rapidly flowing air currents flowing in circular fashion inside a mowing deck largely nullifies the aerodynamic and cutting elements designed into these blades. Evidence to this is found under Pat. No. 1,061,206 dated May 6, 1913 to Nikola Tesla describing a turbine employing smooth circular disks that efficiently capture energy from moving gaseous fluids by “skin resistance.” Since a disk can capture energy, a powered rotating disk can also impart energy into a surrounding fluid. Additionally, rotary mowers employing circular disk blades do not provide an easy escape path for vegetation clippings which promotes clogging of the mowing deck, especially in moist conditions. A circular disk blade is more massive and therefore more expensive to produce which also accounts for its lack of acceptance in the marketplace.
A similar airflow problem exists in Canadian Patent 2,259,040 to Meinerding. A feature of lifting vegetation to be cut by secondary cutting edges installed directly behind a primary cutting surface is shown in FIG. 1, detail 42, and 43. These features are too small to be aerodynamically effective and are mounted directly forward of the vertical shredding members FIG. 1, detail 48, further impeding free airflow. Our tests conclude that this complex, expensive to produce design, also suffers from sever clogging in all but the most dry mowing conditions.
Canadian Patent 2,165,845 to Parkinson employs multiple cutting edges of varying heights, however the design spaces out the cutting members. Many other patents feature cutting members that are substantially spaced apart. Large spacing between cutting members thereby requires the blade to rotate a relatively large angle before the next cutting member will interact with vegetation. For maximum mulching and cutting action, primary and secondary blade members should be grouped closely together, requiring a minimum of blade rotation to complete the cutting action of all interacting blade members. Canadian Patent 2,165,845 to Parkinson also suffers from a common problem associated with all complex designs, which mandates more intricate manufacturing and material requirements which drive costs, and thereby prices well out of the range acceptable in the marketplace.